重庆南川地区中二叠统茅口组眼球状灰岩成因

中二叠世茅口组中-下部发育一种特殊的眼球状灰岩, 眼球部分通常为颜色较浅, 富含生屑颗粒的灰岩; 而眼皮部分通常为颜色较暗的泥灰岩。其成因一直存在分歧。本文通过对重庆南川大铺子剖面茅口组底部眼球状灰岩的野外观察和室内研究, 认为陆源输入的改变导致眼球和眼皮部分泥质含量的差异; 在成岩过程中, 碳酸盐沉积物为主的眼球部分因为胶结作用而没有被机械和压实作用破坏, 泥质含量较多的眼皮部分则受到强烈的压实。因此, 该地区眼球状灰岩的形成是沉积和成岩过程相互作用的结果, 为浅海陆棚沉积环境。

The origin of eyeball-shaped limestone from Maokou Formation (Mid-Permian) in Nanchuan region,Chongqing, Southwest China

During the Mid-Permian Maokouan stage, the Middle-Upper Yangtze region was covered by carbonate rocks. One kind of special structure of these carbonate rocks is named eyeball-shaped limestone, composed of the "eyeball" and "eyelids" components. It is widely distributed in the mid-lower part of Maokou Formation in Sichuan Basin. "Eyeball" is lighter in color, enriched in bioclastic, while "eyelids" are darker in color, dominated by marl. Through field observations and laboratory studies on a typical eyeball-shaped limestone from Maokou Formation at Dapuzi section, Nanchuan, Chongqing. We proposed that a shift in environment deposited the "eyeball" and "eyelids" might have been resulted in the difference in the precursor sediments. During the early diagenesis, carbonate dominated sediments were cemented and prevented from destroyed by mechanical and chemical compaction. In contrast, "eyelids" layers were compacted due to insufficient cementation. Thus, the origin of the eyeball-shaped limestone in this study was attributed to the combined effects of sedimentary and diagenetic processes, and formed shallow shelf sedimentary environments.